Satellite antennas using reflectors for gain and multiple feeds in the configuration of single-feed-per-beam (SFPB) or multiple-feeds-per-beam (MFPB) to produce contiguous spot beam patterns or area coverage patterns have a limitation in which the feed aperture area can be insufficient to illuminate the reflector efficiently. In general, the spillover energy may exceed the optimum value that can be achieved by a single feed sized to provide a net optimum efficiency. In other words, the reflector aperture can be over illuminated and the energy radiated by the feed spilling past the reflector boundary can be greater than the optimum for net efficiency.
The over illumination condition can exist over the practical ranges of focal length values, and generally applies to single reflector optics and to dual-reflector optics. The over illumination condition exists for transmission type convergent optics (e.g., lens) as well as reflector convergent optics. Convergent optics captures radio frequency (RF) energy over a defined area and redirects the energy to a smaller area. The over illumination condition can occur for defocused or focused positions of feeds arranged in a contiguous manner to form contiguous spot beams with reasonable gain loss at the secondary pattern two-beam and three-beam cross-over locations. A similar over illumination condition may arise in the case of an MFPB configuration, where the reflector or lens feeds are defocused to configure a phased array fed reflector antenna.
An approach in SFPB spot beam satellite system applications to improve the illumination uses multiple reflectors for a congruent coverage area and assigns near focused feeds to reflectors in a manner to avoid having contiguous coverage beams within a single reflector.
Another solution uses feed clusters (e.g., 3, 7, 13 elements) and relatively complex orthogonal waveguide beamforming networks to provide overlapping excitation of adjacent feeds to form each beam.
Mitigation examples exist for the over-illumination condition, in which the modes within a feed horn are controlled in an attempt to produce a near uniform amplitude distribution at the horn aperture. In these mode control examples, the near uniform amplitude distribution can be an approximation to the TEM mode in the feed horn structure. Another mitigation example maximizes the feed aperture area in a triangular feed lattice and uses horns having a hexagonal shaped boundary. Neither of these configurations provides optimum illumination conditions and may exhibit only marginal performance improvements over the more common geometry limited configurations.